My proposed research is on the cellular electrophysiology of developing chick and rat hearts and cultured heart cells. Hearts at different stages of embryonic and early post-natal development will be studied with intracellular microelectrode techniques for determining changes in their electrical properties. The properties to be examined include: resting potential, rate of rise and overshoot of the action potential, sensitivity to channel-specific blocking agents, inactivation potentials, membrane resistivity, ratio of Na ion to K ion permeabilities, and intracellular ion concentrations. We will determine exactly when and under what circumstances fast Na ion channels first appear and how they increase in density during development. The normal control exerted over these changes will be evaluated. For example, the innervation will be prevented from reaching young hearts developing in situ. Young hearts will be removed before innervation and organ cultured in glassware and on the chick chorio-allantoic membrane (for blood perfusion) to study whether electrical differentiation of the sarcolemma will occur in vitro and what factors influence this process. For example, we will ascertain the role of protein synthesis by use of inhibitors. Myocardial cells from hearts at different stages of development will be placed into monolayer cell culture and into aggregate cultures to study differentiation of membrane properties in vitro, and the factors in influencing this process. The reversion in electrical properties that occurs when old embryonic cells are placed into cell culture will be further studied in order to identify the factors responsible for this partial dedifferentiation. Voltage clamp experiments will be done on the cultured heart cells to further characterize the slow Na plus channels naturally found and the slow Ca ions Na ion channels induced by catecholamines or cyclic CMP. If heart cells in monolayer culture can be produced which remain highly differentiated, this would give us an excellent preparation with which to answer some of the fundamental questions on cardiac muscles. In summary, we will investigate the changes in electrical properties and functions of the myocardial cell membrane during development and in culture, and the factors influencing these changes.